Recently, silicon base semiconductor devices are increasingly improved in performance as their design rule is miniaturized. It is then critical how to dissipate heat from discrete transistors and metal interconnects between transistors. To address the problem, several measures are taken wherein after device fabrication, the silicon substrate on the back surface is thinned to one to several hundreds of microns, and a large fan is mounted on the chip to promote heat dissipation, or a water-cooled tube is arranged therearound.
However, even when the silicon substrate is actually thinned, the region where the device is fabricated extends several microns from the surface, and the remaining region serves as a heat sump and remains less efficient from the aspect of heat dissipation. Also, SOI wafers used in high-performance processors or the like have the structure including an insulating layer of SiO2 interposed immediately below a device active layer, which structure raises a serious problem from the aspect of heat dissipation because SiO2 has a low heat conductivity of 1.38 W/m·K. Further, silicon substrates suffer a substantial loss in the high-frequency region due to their dielectric properties, with their use being limited.
The silicon-on-sapphire (SOS) technology using sapphire substrate is considered attractive because of efficient heat conduction and low losses in the high-frequency region, but suffers from the following problems. One problem is that since the sapphire substrate is transparent in the visible light region, it is not detected by the optical sensor used for the confirmation of a substrate or for the alignment of a wafer during the device fabrication process. Another problem is that sapphire substrates are expensive.
Exemplary of the substrate which is opaque to visible light, heat conductive and inexpensive are sintered ceramic bodies such as silicon nitride and aluminum nitride. Since these materials are obtained by sintering silicon nitride or aluminum nitride powder together with sintering aids, metal impurities like Fe and Al in the powder or sintering aids like alumina themselves become a cause of contamination to the device fabrication process. These materials are thus awkward to use.
It is noted that Patent Document 1 (JP-A H04-82256) discloses a substrate having a SiO2 film deposited thereon by CVD method for the purposes of insulation and anti-contamination. No reference is made to volume resistivity.